Superintendent Michael Fitzgerald talks to the media about an alleged assault which left a man with critical head injuries outside Kings Cross Police station. Photo: Daniel Munoz

The Commander of Kings Cross Police, Superintendent Michael Fitzgerald, said the brothers and their father were on holiday from Ireland.

"This is a family tragedy. This has nothing to with the lockout [laws] or early closing times."

He said the two brothers got into an argument and it appeared one had punched the other in the head, causing him to "fall to the ground".

Superintendent Michael Fitzgerald said the men's father was in shock.

The pain in his eyes, you could not describe it

"The pain in his eyes, you could not describe it," he said.

"Police are assisting him and are with him at the moment.

"Hopefully he'll have an opportunity to talk to his son before we take him off to court and he'll remain at the bedside for his other son."

The injured sibling was taken to St Vincent's Hospital where he is now undergoing emergency surgery and remains in a critical condition.

Superintendent Fitzgerald said "fingers and crossed, prayers are out there" for the young man to survive.

He said it was not yet clear which part of his head had been hit.

"From what I'm told, we won't know anything for at least 24 to 48 hours. They'll release pressure on the brain."

There were a number of witnesses to the fight, which was also captured on CCTV.

"It is quite evident that this resulted from a minor argument," Superintendent Fitzgerald said.

He said it appeared from the footage that the victim had pushed his brother and "the brother has reacted".

The fight took place in Bayswater Road after the father and sons had been out to dinner.

"Then he (the father) went home and I believe they went out and had a drink afterwards," he said.

Asked whether the father had asked police not to charge his son with recklessly causing grievous bodily harm, Superintendent Fitzgerald said: "He wouldn't ask that and we can't comply with it either."

He said "the aggravating factor of alcohol was investigated and has been negated."

In January 2014, the NSW government introduced lock-out laws in Sydney's CBD entertainment precinct following the deaths of two young men caused by "one-punch" assaults. The entertainment precinct stretches from parts of Surry Hills and Darlinghurst to The Rocks, and from Cockle Bay to Kings Cross.

The lockout laws were relaxed on New Year's Eve but have applied as normal since New Year's Day.